11,252 research outputs found
INTEGRAL and Swift/XRT observations of IGR J19405-3016
IGR J19405-3016 is reported in the 3rd IBIS catalog as being one of its
lowest significance sources (~4.6 sigma under an exposure of about 371 ks).
This leads to a caveat in multi-wavelength study although the source was
identified in optical as a Seyfert 1. The currently increased INTEGRAL data
stimulate us to investigate the reality of this source. We analyze all
available observations carried out by INTEGRAL and Swift on IGR J19405-3016. We
find that IGR J19405-3016 has a detection significance of ~ 9.4 sigma in the
20-60 keV band during the observational period between March 2003 and March
2008. Thus confirms a real source detection reported previously. The source
position and error location are therefore updated. The source is found to be
constant over years at the hard X-rays. Over the three XRT observations, the
source flux varied by up to 39% from the average, and the spectrum is generally
soft. The combined XRT/ISGRI spectrum is well fitted with a simple power law
model (photon index 2.11+-0.03). Such a photon index is well consistent with
the mean value 1.98 (dispersion 0.27) as obtained from Swift/BAT AGN samples at
14-195 keV. The spectral slope of IGR J19405-3016 is larger than the average
spectral slope found by Molina et al. (2009). A similar discrepancy is found
with the results of Beckmann et al. (2009) regarding Seyfert 1 AGNs. A possible
explanation of this simple spectral description may be that the low level of
the column density allows for the `true' spectrum to appear at soft X-rays as
well.Comment: Astronomy and Astrophysics, in pres
Non-Perturbatively Improved Quenched Hadron Spectroscopy
We make a quenched lattice simulation of hadron spectroscopy at beta=6.2 with
the Wilson action non-perturbatively improved. With respect to the unimproved
case, the estimate of the lattice spacing is less influenced by the choice of
input hadron masses. We study also the effects of using an improved quark mass
in the fits to the dependence of hadron masses upon quark masses.Comment: 12 pages, including 5 postscript figure
The prompt optical/near-infrared flare of GRB 050904: the most luminous transient ever detected
With a redshift of z=6.295, GRB 050904 is the most distant gamma-ray burst
ever discovered. It was an energetic event at all wavelengths and the afterglow
was observed in detail in the near-infrared bands. We gathered all available
optical and NIR afterglow photometry of this GRB to construct a composite NIR
light curve spanning several decades in time and flux density. Transforming the
NIR light curve into the optical, we find that the afterglow of GRB 050904 was
more luminous at early times than any other GRB afterglow in the
pre-\emph{Swift} era, making it at these wavelengths the most luminous
transient ever detected. Given the intrinsic properties of GRB 050904 and its
afterglow, we discuss if this burst is markedly different from other GRBs at
lower redshifts.Comment: The Astronomical Journal, in press; revised version, including the
comments of the referee (one figure added, text restructured, all conclusions
unchanged), 7 pages, 3 figure
Resolving Gamma-Ray Burst 000301C with a Gravitational Microlens
The afterglow of the Gamma-Ray Burst (GRB) 000301C exhibited achromatic,
short time-scale variability that is difficult to reconcile with the standard
relativistic shock model. We interpret the observed light curves as a
microlensing event superimposed on power-law flux decays typical of afterglows.
In general, a relativistic GRB shock appears on the sky as a thin ring
expanding at a superluminal speed. Initially the ring is small relative to its
angular separation from the lens and so its flux is magnified by a constant
factor. As the ring grows and sweeps across the lens its magnification reaches
a maximum. Subsequently, the flux gradually recovers its unlensed value. This
behavior involves only three free parameters in its simplest formulation and
was predicted theoretically by Loeb & Perna (1998). Fitting the available
R-band photometric data of GRB 000301C to a simple model of the microlensing
event and a broken power-law for the afterglow, we find reasonable values for
all the parameters and a reduced chi^2/DOF parameter of 1.48 compared with 2.99
for the broken power-law fit alone. The peak magnification of ~2 occurred 3.8
days after the burst. The entire optical-IR data imply a width of the GRB ring
of order 10% of its radius, similar to theoretical expectations. The angular
resolution provided by microlensing is better than a micro-arcsecond. We infer
a mass of approximately 0.5 M_Sun for a lens located half way to the source at
z_s=2.04. A galaxy 2'' from GRB 000301C might be the host of the stellar lens,
but current data provides only an upper-limit on its surface brightness at the
GRB position.Comment: to appear in the ApJ Letters, 13 pages, 3 figures (one additional
figure included); all data used for the fits available at
ftp://cfa-ftp.harvard.edu/pub/kstanek/GRB000301C/ and through WWW at
http://cfa-www.harvard.edu/cfa/oir/Research/GRB
Constraining and Dark Energy with Gamma-Ray Bursts
An relationship with a small
scatter for current -ray burst (GRB) data was recently reported, where
is the beaming-corrected -ray energy and
is the peak energy in the local observer frame. By considering this
relationship for a sample of 12 GRBs with known redshift, peak energy, and
break time of afterglow light curves, we constrain the mass density of the
universe and the nature of dark energy. We find that the mass density
(at the confident level) for a flat
universe with a cosmological constant, and the parameter of an assumed
static dark-energy equation of state ().
Our results are consistent with those from type Ia supernovae. A larger sample
established by the upcoming {\em Swift} satellite is expected to provide
further constraints.Comment: 8 pages including 4 figures, to appear in ApJ Letters, typos
correcte
Unveiling the nature of the unidentified gamma-ray sources III: gamma-ray blazar-like counterparts at low radio frequencies
About one third of the gamma-ray sources listed in the second Fermi LAT
catalog (2FGL) have no firmly established counterpart at lower energies so
being classified as unidentified gamma-ray sources (UGSs). Here we propose a
new approach to find candidate counterparts for the UGSs based on the 325 MHz
radio survey performed with Westerbork Synthesis Radio Telescope (WSRT) in the
northern hemisphere. First we investigate the low-frequency radio properties of
blazars, the largest known population of gamma-ray sources; then we search for
sources with similar radio properties combining the information derived from
the Westerbork Northern Sky Survey (WENSS) with those of the NRAO VLA Sky
survey (NVSS). We present a list of candidate counterparts for 32 UGSs with at
least one counterpart in the WENSS. We also performed an extensive research in
literature to look for infrared and optical counterparts of the gamma-ray
blazar candidates selected with the low-frequency radio observations to confirm
their nature. On the basis of our multifrequency research we identify 23 new
gamma-ray blazar candidates out of 32 UGSs investigated. Comparison with
previous results on the UGSs are also presented. Finally, we speculate on the
advantages on the use of the low-frequency radio observations to associate UGSs
and to search for gamma-ray pulsar candidates.Comment: 15 pages, 13 figures, 3 tables, ApJS accepted for publication
(version pre-proof corrections
IGR J00234+6141 : a new INTEGRAL source identified as an Intermediate polar
Following an extensive survey of the galactic plane by the INTEGRAL
satellite, new hard X-ray sources are discovered with a significant fraction of
Cataclysmic Variables (CVs) among them. We report here the identification of
one of these hard X-ray sources, IGR J00234+6141, as an accreting magnetic
white dwarf of intermediate polar type. We analyse the high energy emission of
the INTEGRAL source using all available data and provide complementary optical
photometric and spectroscopic data obtained respectively in August and October
2006. Based on a refined INTEGRAL position, we confirm the proposed optical
identification. We clearly detect the presence of a 564 s periodic optical
modulation that we identify as the rotation of the white dwarf. The analysis of
the optical spectrum also demonstrates that the emission lines show a
modulation in radial velocity with an orbital period of Porb = (4.033 +/-
0.005) hr. The two periodicities indicate that IGR00234+6141 is a magnetic CV
of the intermediate polar type. This is one of the faintest and hardest sources
of this type detected by INTEGRAL. This confirms earlier conclusions that IPs
contribute significantly to the population of galactic X-ray sources and
represent a significant fraction of the high energy background.Comment: 6 pages, 7 figures, accepted for publication in A&
INTEGRAL/IBIS nine-year Galactic Hard X-Ray Survey
Context. The INTEGRAL observatory operating in a hard X-ray/gamma domain has
gathered a large observational data set over nine years starting in 2003. Most
of the observing time was dedicated to the Galactic source population study,
making possible the deepest Galactic survey in hard X-rays ever compiled. Aims.
We aim to perform a Galactic survey that can be used as the basis of Galactic
source population studies, and perform mapping of the Milky Way in hard X-rays
over the maximum exposure available at |b|<17.5 deg. Methods. We used sky
reconstruction algorithms especially developed for the high quality imaging of
INTEGRAL/IBIS data. Results. We present sky images, sensitivity maps, and
catalogs of detected sources in the three energy bands 17-60, 17-35, and 35-80
keV in the Galactic plane at |b|<17.5 deg. The total number of sources in the
reference 17-60 keV band includes 402 objects exceeding a 4.7 sigma detection
threshold on the nine-year time-averaged map. Among the identified sources with
known and tentatively identified natures, 253 are Galactic objects (108
low-mass X-ray binaries, 82 high-mass X-ray binaries, 36 cataclysmic variables,
and 27 are of other types), and 115 are extragalactic objects, including 112
active galactic nuclei (AGNs) and 3 galaxy clusters. The sample of Galactic
sources with S/N>4.7 sigma has an identification completeness of ~92%, which is
valuable for population studies. Since the survey is based on the nine-year sky
maps, it is optimized for persistent sources and may be biased against finding
transients.Comment: 18 pages, 7 figures, long table, Astronomy and Astrophysics 545, A2
IGR J18483-0311: an accreting X-ray pulsar observed by INTEGRAL
IGR J18483-0311 is a poorly known transient hard X-ray source discovered by
INTEGRAL during observations of the Galactic Center region performed between
23--28 April 2003. Aims: To detect new outbursts from IGR J18483-0311 using
INTEGRAL and archival Swift XRT observations and finally to characterize the
nature of this source using the optical/near-infrared (NIR) information
available through catalogue searches. Results: We report on 5 newly discovered
outbursts from IGR J18483-0311 detected by INTEGRAL.For two of them it was
possible to constrain a duration of the order of a few days. The strongest
outburst reached a peak flux of 120 mCrab (20--100 keV): its broad band
JEM--X/ISGRI spectrum (3--50 keV) is best fitted by an absorbed cutoff power
law with photon index=1.4+/-0.3, cutoff energy of ~22 keV and Nh ~9x10^22
cm^-2. Timing analysis of INTEGRAL data allowed us to identify periodicities of
18.52 days and 21.0526 seconds which are likely the orbital period of the
system and the spin period of the X-ray pulsar respectively. Swift XRT
observations of IGR J184830311 provided a very accurate source position
which strongly indicates a highly reddened star in the USNO--B1.0 and 2MASS
catalogues as its possible optical/NIR counterpart. Conclusions: The X-ray
spectral shape, the periods of 18.52 days and 21.0526 seconds, the high
intrinsic absorption, the location in the direction of the Scutum spiral arm
and the highly reddened optical object as possible counterpart, all favour the
hypothesis that IGR J18483-0311 is a HMXB with a neutron star as compact
companion. The system is most likely a Be X-ray binary, but a Supergiant Fast
X-ray Transient nature can not be entirely excluded.Comment: accepted for publication in A&A, 10 pages, 17 figures, 4 table
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